Electrodes for batteries are roughly classified into three types: the paste type electrode in the form of a lattice or net serving as an active material supporting body and having a pasty active material filled thereinto, the sintered type electrode in the form of a sintered metal base plate with an active material filled into its micropores, and the pocket type electrode in the form of a microporous tube or pocket with a powdery active material filled thereinto.
The paste type electrode is well-known as a lead storage battery electrode and in some cases it is also used as a cadmium electrode for nickel-cadmium storage batteries. Although this electrode is relatively easy to produce, its pasty active material tends to come off and the ruggedness of the electrode is not satisfactory. The pocket type electrode involves a problem that it is lower in active material utilization percentage than the paste type electrode and sintered type electrode.
The sintered type electrode is superior to the other two types with respect to its electrode characteristics and ruggedness. However, the sintered base plate is an expensive material and the operation of filling a necessary amount of active material into the micropores of the base plate makes it necessary that immersion in an active material solution and thermal decomposition be performed several times and that the base plate be immersed in such active material solution to effect electrolytic deposition; thus, as compared with the other types, it requires a complicated process for its fabrication, and the cost of this type of electrode is the highest.
Recently, there has been proposed an electrode, similar to the sintered type electrode, using a foamed porous metal body (hereinafter referred to as sponge metal) as a base body with a pasty active material filled thereinto. The sponge metal has a three-dimensional reticulated structure, a porosity of as high as 95%, and a large pore diameter, and during production the pore diameter can be optionally selected. Therefore, by suitably selecting the pore diameter it is possible to fill a pasty active material or a powder of active material directly into the pores and hence this electrode is characterized in that it can be produced in a much simpler process than in the case of the sintered type electrode.
Among the practical methods of filling an active material into this sponge metal are a method which imparts vibrations to the pasty active material, a method which rubs in the pasty active material, a method which contacts the pasty active material with one surface of the sponge metal while applying suction to the other surface so as to fill it thereinto, and a method which sprays the pasty active material onto the sponge metal through a nozzle.
Of these methods, the rubbing method and the spraying method are suitable for uniform filling of an active material into the pores of the sponge metal. A comparison of these two methods shows that the spraying method, in which there is no possibility of the nozzle coming in contact with the sponge metal, is superior to the rubbing method in the durability and ease of maintenance of the device used for filling.
Such a spray method of filling a pasty active material into sponge metal is disclosed in Japanese Patent Publication No. 20664/1981. The spray method, though having merits concerning the device, results in some difficulty in filling a pasty active material so that it has a uniform composition throughout the sponge metal.
That is, if a pasty active material is simply supplied at a given flow rate and sprayed by a nozzle, the non-uniformity of dispersion of the powder particles (solid component) in the paste results in the non-uniformity of the composition of the paste after it has been filled into the pores of the sponge metal, or too much air entering the paste results in a sponge cake form, making substantial filling of the active material impossible or greatly increasing the variation of the filling.
Further, if the spray nozzle is positioned merely at one side of the sponge metal and a pasty active material is sprayed, a paste portion containing a relatively large amount of powder will be filled into the sponge metal surface nearer to the nozzle but a paste portion containing less powder than the paste portion on said surface penetrates into the center of the thickness of the sponge metal and the back surface of the sponge metal farthest from the nozzle will be filled only with a paste portion having a greater amount of the binding agent solution having a greater penetrating ability than the powder and will have little powder content, or the passage of the paste is impeded and hence filling is substantially impossible. Therefore, the method is unsatisfactory from the standpoint of filling a preadjusted composition paste into the entire volume sponge metal with good uniformity.